The concentrations of metal ions in various media are extensively measured both in the biological sciences and in the health care industry. Several methodologies exist for taking these measurements, including the use of ion selective electrodes, ion responsive dyes, and ion sensitive field effect transistors (IFETS). (See, e.g., Supramolecular Chemistry I--Directed Synthesis and Molecular Recognition, Weber, E., Ed., Springer-Verlag, New York, 1993.) However, ion selective electrodes have the disadvantage that they are not easily miniaturized, and IFETS and ion responsive dyes lack the high sensitivity necessary for trace analysis.
Conducting polymers (CPs) have been the focus of considerable interest because they combine the relatively low cost and ease of manufacturing of polymers with the conductive properties of metals and semiconductors. Moreover, the conductivity of conducting polymers is highly responsive to both conformational and electrostatic perturbations. For example, it is well known that twisting a conducting polymer's backbone from planarity can result in a conductivity drop as high as 105 or greater. See, e.g., Handbook of Conducting Polymers, Skotheim, T. J., Ed., Dekker, New York, 1986. Hence, conductivity changes in conducting polymers provide a large dynamic range which, if harnessed effectively, can result in very sensitive sensory materials. Such conductivity changes easily can be monitored and miniaturized. See, e.g., Kittlesen, et al., J. Am. Chem. Soc. 1984, 106, 7389.
Conducting polymer-based sensors have been previously reported. (See, e.g., Thackeray, et al., J. Phys. Chem. 1986, 90, 6674; Zotti, Synthetic Metals 1992, 51, 373.) However, known polymer-based sensors are chemically irreversible and cannot detect a time dependent signal in real time. This is a serious deficiency where it is desired to measure stimuli which vary over time, such as in the monitoring of electrolyte concentrations in bodily fluids. Additionally, there are no systems at present that can be easily modified to detect a variety of chemical species.
Substituted polythiophenes are an ideal choice for sensory materials due to their ease of structural modifications, high conductivity, and environmental stability. In addition, recent studies have shown the conductivity of these materials to be highly sensitive to the nature and regiospecificity of covalently bound sidechains, indicating that small conformational changes produce large effects. (See, e.g., Roncali, J. Chem. Rev. 1992, 92, 711; Heywang, et al., Adv. Mater. 1992, 4, 116; McCullough, et al, J. Am Chem. Soc. 1993, 115, 4910.) However, previous attempts to develop polythiophene-based sensory materials showed no ion-selective electrochemical response. (see, Sable, et al., Electrochemica Acta 1991, 36, 15.)
Consequently, there remains a need in the art for conductive polymers whose conductivities change reversibly in response to a variety of chemical species.